Separation of aromatic amines



Nov. 24, 1953 Filed April 6 1950 A. G. HILL. Erm. 2,660,581

SEPARATION OF AROMATIC AMINES 2 Sheets-Sheet l ATTOR N EY Nov. 24, 1953A, G, HlLL ETAL 2,660,581

SEPARATION OF' AROMATIC AMINES Filed April 6 1950 2 Sheets-Sheet 2ATTORN EY Patented Nov. 24, 1953 stasi SEPARATION OF AROMATIC AMINESAlfred G. Hill, Martinsville, Eugene C. Medcalf,

Bound Brook, and William E. Sisco, Somerville, N. J., assignors toAmerican Cyanamid Company, New York, N. Y., a corporation of MaineApplication April 6, 1950, Serial No. 154,364

18 Claims.

This invention relates tc the separation of aromatic amines havingdifferent carbon to hydrogen ratios by fractional extraction.

Various substituted aromatic amines are produced by processes whichresult in mixtures difcult to separate. For example, when .aromaticcarbocyclic amines are alkylated it is practically impossible to producea pure N-monoalkyl aromatic amine; it is always mixed with N,Ndialkylaromatic amine and sometimes with a position isomer in'which part oialkyl groups attach themselves to ring carbon atoms. The close boilingpoints of these mixtures often preclude simple separation by fractionaldistillation and in the past it has been necessary to use chemicalseparation methods, for example treatment with an acid `anhydride andthe like. These chemical separa-tion methods are relatively costly andcontribute greatly to the high cost of many monoalkyl aromatic amines.

Carbocyclic aromatic amines have been referred to above. The process ofthe present invention, however, is not limited to the separation of suchamines where the amino group forms no part of the ring. On the contrary,excellent separations may be obtained between heterocyclic tertiaryamines of different carbon to hydrogen ratio belonging to the pyridine,quinoline, isoquinoline, pyrimidine, pyrazine and other heterocycliccompounds.

A new class of solvents containing two nitrile groups has been developedfor the separation of unsaturated hydrocarbons such as aromatics fromvarious petroleum fractions. Typical of these solvents are thebis(2cyanoallyl) ethers, The effectiveness of the show any difference insaturation; that is to say.

the number of double `bonds remains the same. It is therefore notsurprising that the dinitrile solvents can not be used to separate thevarious amines by a simple leaching process because, in

most cases, both amines are quite soluble in the dinitrile solvents. Insome cases both amines are miscible substantially in all proportions.

' The process of the present invention is one oi fractional extraction.We have found that if mixtures of aromatic amines of different carbon tohydrogen ratios are subjected to the action oi a mixture of a dinitrilesolvent and a saturated hydrocarbon liquid, for example bycounter-current extraction in suitable extraction columns, it ispossible to effect good separation in spite of the fact that both aminesare quite soluble in both solvents. In the presence of the two solvents,the amines having higher carbon to hydrogen ratio concentrate in thedinitrile solvent, and those having a lower carbon to hydrogen ratio, inthe saturated hydrocarbon. Satisfactory separations are obtainable withva reasonable extraction cycle and with a reasonable amount of recyclingof solvents. Losses are extremely low, as the solvents can be recoveredalmost quantitatively and for the first time there is available a cheapprocess for obtaining monoalkyl aromatic amines by direct alkylation incases where monoand dialkylamines do not have a sufficiently greatdifference in boiling points to separate them readily by distillation.

The dinitrile solvent may be represented by the following formula:

oNAik oNAlk in which Alk is a lower alkylene radical and X is selectedfrom the group consisting of oxygen, sulfur, NH and N-lower alkyl. Amongthe ethers, bis(2,cyanoethyl)ether is the cheapest and the mosteffective. Other ethers, such as dicyanodimethyl ether,a,'dicyanodiethyl ether, various isomeric dicyanodipropyl ethers, mixedethers such as cyanoethyl-cyanomethyl ether, and the like, may be used.The cheapest dicyanodialxyl amine is bis(2rcyanoethyl) amine. ther'amines may be used, such as bis(cyanomethyl)N-meth ylamine,a,'dicyanodiethyl amines, bis cyanomethyDa-mine, various dicyanoisomeric dipropyi ramines and the like. Among the sulfldes, bis@-cyanoethyDsulde is the most useful.

As a second solvent, any saturated hydrocarbon liquid can be used, suchas n-heptane, dodecane, aliphatic naphthas which are mixtures of parar'-ns having from s ix to eight carbon atoms, cycloparains such ascyclohexane, methylcyclohexane and the like. The choice of saturatedhydrocarbon liquid is largely governed by its boiling point, because inmany cases a saturated hydrocarbon may be chosen which boils at atemperature so far removed from the boiling point of the amine, which isextracted, that cheap and simple separation by distillation can beeffected. rfhere is, naturally, a certain small difference in relativesolubilities of the amines having higher hydrogen to carbon ratio in thedifferent saturated hydrocarbons; however, this solubility difference isusually not so great as to dictate the choice of hydrocarbon to use; theother -factors, such as its boiling point, often being decisive.

The unexpected ability to effect sharp separation of mixtures, ofamines, both of which are quite soluble in the dinitrile solvents, doesnot, however, result in a mechanism which fails to follow basic physicallaws as applied to fractional separation processes, and, in common withthese processes, there is an exponential factor making it ordinarilyuneconomic to obtain a complete separation with some amine mixtures anda suitable choice of dinitrile solvent and saturated hydrocarbon. It ispossible with modern equipment and satisfactory output to obtain aseparation in which each constituent separated contains as little as 2%of the other. In most cases, however, a separation of 95% and 5% areconsidered adequate. This degree of separation will ordinarily besufficient to permit use of the amine without further purification. In afew cases where complete separation is required, the products of theprocess of the present invention may be subjected to further separationby chemical means; in such cases, however, the consumption of chemicalwhich renders chemical separation very expensive or prohibitive, isreduced to such a small fraction that economic production of pure aminesbecomes feasible. In other words, the present invention in given casesmay be used as the only method of purification, whereas in other casesit is associated with different purification methods but greatly reducesthe cost.

It is possible to use the present invention in a series of batchseparations; however, economic separation on a large scale usuallyrequires a continuous procedure. We therefore prefer to usecounter-current extraction columns, though the invention in its broaderaspects is not limited thereto. Extraction requirements are usuallycalculated in terms of theoretical plates, though frequently a platecolumn is less suitable for extraction than other types'of vessels. Asin all extraction processes. the degree of separation is a product of.two factors, the plate equivalent of the column and the amount of therecycling of the two solvents, which, in analogy to distillationprocedures, is ordinarily referred to as reflux Commercial operation isalways a compromise between equipment cost and output. The moreequivalent plates a column has, the more expensive it is, the less isthe reiiux required, and therefore output per unit of time is increased.It is customary in extraction processes to multiply the theoreticalplates by a factor which will give the column size for commerciallypractical reux ratios. When this factor is applied, the column is stillrated in terms of theoretical plates, which define its actual size. Inthe present case a factor of two has been used for all of the examples.

The dinitrile solvents, which in the drawings described below arereferred to as SN, are ordinarily used as single solvents becauseusually this is the form in which they are commercially produced. Itshould be understood, however,`

that mixtures of dinitrile solvents may also be used. The saturatedhydrocarbon solvent, however, which is designated on the drawings as SH,will frequently be used in the form of a mixture because it is usuallycheaper to obtain a naphtha fraction than pure n-heptane. It is anadvantage of the present invention that Dure hydrocarbons areunnecessary and that readily obtainable commercial mixtures may be used.

While the present invention is not primarily concerned with theprocesses used in recoverinsr `the solvents, i. e. separating thedissolved amines therefrom, it is an advantage of the invention thatvery effective separation processes are available. The saturatedhydrocarbon solvent is almost always recovered by distillation, for itis completely stable under ordinary distilling conditions and this cheapmethod is ordinarily preferred.

The recovery of the dinitrile solvent, on the other hand, presentssomewhat more of a problem. Many of these solvents are not stable atclistilling temperatures and this is one of the reasons why they havenot achieved hitherto extensive commercial use in the separation ofaromatics from petroleum fractions. There have been developed, however,very effective processes for the recovery of dinitrile solvents. One ofthese processes which is useful with practically all of the dinitrilesolvents, except bis(2cyano ethyl) amine, involves the use of water.Most of the dinitrile solvents have extraordinarily steep temperaturecurves of solubility in water. They can, therefore, be extracted orseparated by cooling. The solubility at room temperature, except in thecase of the bis 2cyanoethyl) amine referred to above, is so low that asharp separtion results. The small amount of dinitrile in the waterlayer is not lost since the water can be reused, and the small amount ofwater in the dinitrile solvent does not adversely affect its action inthe main fractional extraction process of the present invention. Therecovery by means of water is not claimed here but constitutes thesubject matter of the copending applications of Medcalf and Sisco,Serial Nos. 143,585 now Patent No. 2,568,159 and 143,586 now Patent No.2,568,160, both filed February 10, 1950.

Another method of recovering dinitrile solvents which is applicable toall of them is by extraction with a solvent, such as a saturatedhydrocarbon, which removes the dissolved amine without dissolvingsignificant amounts of the dinitrile solvent. Any slight amountsdissolved are of no significance because the hydrocarbon solvent isreused in the main p-rocess. This separatory process is described andclaimed in the copending application of Vriens, Sisco and Medcalf,Serial No. 143,584, led February 10, 1950 now Patent No. 2,568,176.

The invention will be described in greater detail in the followingspecific examples and in the drawings, in which:

Fig. 1 is a diagrammatic representation of a process using water fordinitrile solvent separation, and

Fig. 2 is a diagrammatic representation of a process using a hydrocarbonextraction method of recovering dinitrile solvents.

Example 1 This example relates to the separation of a mixture ofN-monomethylaniline and N,Ndi-

methylaniline, by the methodshown in Fig. 1. The mixture 'of amines,labelled AM and An respectively, is fed into an extraction columnY I ofapproximately six theoretical plates. Bis@- cyanoethyllether, labelledSN, is introduced into the top of the column through pipe 'I from thestorage tank 2 at a rate about ve times the rate of amine mixture feed.At the same time a saturated hydrocarbon, n-heptane, labelled SH, isintroduced into the bottom of the column through pipe ii from itsstorage tank 3 at a rate approximately equal to that of SN. As theheptane is lighter than the bis(2cyanoethyl)ether, it rises, and thelatter sinks, and during the countercurrent ow the two methylanilinesdistribute themselves between the two solvents, the SN preferentiallydissolving AM and the heptane SH preferentially dissolving AD. Thissolution in SH leaves the top of the` column and flows into still dthrough pipe 9 where SH is distilled off, returning to the storage tank3. A stream of the dialkyl aniline AD flows out of the bottom of thestill into a product storage tank 5. A portion of this product isreluxed into the top of the column I through pipes 6 and l.

A relatively rich solution of AM in SN ows out of the bottom of column Iinto heater l where it is heated to about 80 C. and introduced into asolvent recovery extraction column I I. Water,

pre-heated to approximately the same temperature in heater l2, isintroduced into the top of the column and ows down. The water extractsSN, and AM iiows out from the top of the column through pipe I3. Aportion of this stream is refluxed into the extraction column I, throughpipes I4 and 8, the remainder going to product storage tank I 5. Amixture of water and SN ows out of the bottom of column I I into acooler I6 and, after cooling, into a separator Il where it separatesinto an upper water layer and a lower solvent layer. The former flowsthrough pipe I 8 to heater I2, and the latter through pipe I9 intostorage tank 2.

The owsheet of Fig. 1 represents conditions after equilibrium has beenestablished. The

amount of reflux of AD and AM is regulated so that the AD going to tankcontains 5 mol percent of AM, and the AM going to tank I5 about 5 molpercent An.

There is substantially no loss of SH, and SN losses in the tank I5 arevery small. Usually it will not be necessary to wash this product with alittle hot water, although this can be done where the recovered SN isworth the cost. The

separation in separator Il is not 100%, there being a little SN in thewater and a little water in the SN. The former is not lost as it isrecirculated through the extraction column i l, and the latter is toosmall to interfere with the solvent characteristics of SN in extractioncolumn l to any serious extent (not more than one or two theoreticalplate equivalents). If desired, however, the water can be removed byvacuum evaporation. In practical operation with the amine mixture ofthis example, the removal of the water can hardly be justied.

Fig. 1 is a diagrammatic fiowsheet and for clearness the heating andcooling functions have been physically separated; that is to say,separate heaters I0 and l2 and a separate cooler it are shown. In anactual plant, heating and cooling operations are effected in heatexchangers to save fuel costs. The drawing, therefore, should beconsidered as a purely diagrammatic process owsheet.

6 Example 2 In this example a mixture of N-monoethyl-otoluidine andN,N-diethylo-toluidine is separated using bis(2cyanoethyl) amine as SNand, as SH, VMP naphtha, a petroleum fraction contain-l ing parailinshaving from seven to nine carbon atoms, and having a solventcharacteristic substantially the same as n-heptane. The separationprocedure follows the iowsheet of Fig. 2, as the water method ofrecovering SN can not be used with bis(2-cyanoethyl) amine because thelatter is too soluble in cold water. In Fig. 2 the extraction column Iand its associated equipment is identical with that of Fig. 1 exceptthat column I is of four theoretical plates, and the same parts bear thesame reference numerals, the monoand di-ethyl amines being referred toas AM and AD as in the first example. The operation of the firstextraction column proceeds in exactly the same manner as in Fig. 1, andN,Ndiethyl toluidine of approximately 95 mol percent is recovered intank 5.

The mixture of SN and AM leaving the bottom of extraction column I,however, is subjected to a different treatment in order to separate itsconstituents. The stream is introduced at the top of an extractioncolumn 20, into the bottom of which SH from tank 3 is introduced throughpipe 2|. The latter being lighter rises in the column and extracts AMfrom SN, the latter containing only traces of AM leaving the bottom ofthe extraction column and being returned to storage tank 2 through pipeI9. The solution of AM and SH leaves the top of the extraction columnand ilows through pipe 22 into still 24. SH is vaporized and, aftercondensing, ows through pipe 23 back to storage tank 3. The stillresidue, which is mostly AM, is divided into two streams, one going tothe product storage tank 25, and the other being reluxed into extractioncolumn I through pipes 26 and 8.

There is no loss of either SN or SH except mechanical losses which areextremely small.

Eample 3 The procedure of Example l is followed, using a mixture ofaniline as AM and mesidine as An. Extraction column I corresponds tofour to five theoretical plates. The separation is the same as inpreceding examples; that is to say, each product is obtained inapproximately mol percent purity.

Example 5 The fiowsheet of Example 2 is followed withVN-monoethyl-o-toluidine as AM and l\T,N-diethyl o-toluidine as AD.Extraction column I has three to four theoretical plates for theproduction of Q5 mol percent pure products.

Example 6 The procedure of Example 2 is followed in the separation ci amixture of amines in which N- monoethyl-a-naphthylamine is AM and N,N-diethyl-a--naphthylarnine is AD. SN is bis 2 cyanoethyDether and SH isn-heptane. Extrac- Z tion column l has three theoretical plates forproducts or 95 mol percent purity.

Example 7 The procedure of Example 2 is followed in the separation of amixture of amines in which monomethylaniline isy AM andk N,Nd imethylaniline is SMN is bisiZ-cyanoethyl).ether, and SH is VMP naphtha. Theextraction column I isV of eight theoretical plates., and a recovery ofthe products of 98 mol percent purity is obtained.

Example 8 Exemple 1 0 The. procedure of Example is, followed using bis2fcyanoethyll amine as SN and VMI naphtha as SH. The extraction columnhasy four theoretical plates, and products are obtained of 95 molpercent purity.

Err-:imple 1 .l

The procedure of Example l is followed using e'dicyanodiethyl ether asSu and methyl cyclohexane as Sn. rEhe extraction column has seventheoretical plates, and products are obtained having a purity of 95 molpercent.

Efcample 12 The procedure of Example 8 is followed using bis 2cyanoethylether as SN and VMP naphtha as Sn. Four theoretical plates are necessaryto produce products of 98 mol percent purity.

Example 13 A series of separations of heterocyclic tertiary amines arecarried out using bis(2-cyanoethyl) ether as SN and n-heptane as SH. Theprocedure otherwise follows Example 2. In each case the separation waseffected to produce products of 95 mol percent purity. The `followingtable gives the separations with the corresponding theoretical plates ofthe separating column:

Theoretical A I AD plates -picollue1. 18 o. 2,6lutidinc 8 QuinolineQuinaldine -16 Isoquinolind.. do 16 l'ilhevdistricution coefficient foreplcolinc is substantially the same as the mixtures o the three isomericpicolines. The same equipment can, therefore, he used to separate amixture of tbc picolilies from pyridine.

Figs. 1 and 2 show the feed of AM plus AD entering the center of the rstextraction column. rThis position corresponds to a feed of equal partsof the two amines. If the feed contains the amines in differentproportions, the point of introduction into the extraction column willbe different because, for maximum effective 8 utilization ofthe column,the feed should bein-, troduced4 at a point where the. amines are preseent` in substantially the samer proportions as in the feed.

In both Figs. 1 and 2 the extraction column i, and in Fig. 2 also column2e, is shown With SN entering the top of the column and SH the bottom.This is dictated by the relative speeic gravities of the solvents or,more accurately, the specic gravities of the solutions AM and AD in thesolvents. In practically every case, the solution in SN will be heavierand therefore the introduction into the top of the column as shown inthe drawings will bev followed. In the rare exception Where the specificgravities are the reverse, SN is introduced into the bottom of thecolumn and SH into the top. Similarly, in the extraction column il, thewater is introduced at the top because, in that particular example, asolution of SN in water has a higher specic gravity than N-monoethyline,AM. With some amines the reverse is true, in which case the connectionsto the column should be reversed.

It is an advantage of the invention that the ratio of amine feed tosolvents is not critical. However, practical considerations requireoperating with fairly dilute solutions because if theV solution is tooconcentrated, some amines exert a mutual solvent action so that SN andSH no longer are substantially mutually insoluble. After reaching thepoint where the amines exert any substantial mutual solvent action,reduction of the rate of flow of SH and SN will rapidly reduce theefficiency of separation and is therefore undesirable. The examples showa good safe practical concentration for most separations. They are,however, not critical, and somewhat higher concentrations do no harm asfar as the sepaf rating efficiency isV concerned. However, operatingwith very dilute solutions results in a marked lowering of output, andwhile technically en.-V tirely operative, they are undesirable from aneconomic standpoint.

We claim:

1. A method of separating aromatic amines having diierent carbon tohydrogen ratios and containing not more than two rings, which comprisescontacting a mixture of said aromatic amines with a mixture ofimmiscible organic solvents for said aromatic amines, one of saidimmiscible solvents being a dinitrile solvent having the followingformula lNC-Alk-X-Alk-CN in which Alk is a lower alkylene radical and Xis selected from the group consisting of oxygen, sulfur, N H and N-loweralkyl and the other immiscible solvent being a saturated hydrocarbonsolvent, until the mixture of aromatic airlines is partitioned betweenthe said immiscible solvents, separating the immiscible solvent layersso formed, recovering the aromatic amine having the highercarbon tohydrogen ratio from the separated d intrile solvent layer and separatelyrecovering the aromatic amine having the lower carbon to hydrogen ratiofrom the separated hy.- drocarbon solvent layer.

2. A continuous process according to claim 1 in which the partitioningof the mixture of aromatic amines between the said immiscible solvent iscontinuously effected in an extraction column by continuouslyintroducing the immiscible dinitrile and hydrocarbon solvents,respectively, at the proper opposite ends of said column to produce acountercurrent ow of the immiscible solvents relative to each otherthrough the extraction column and continuously feeding the mixture ofaromatic amines into the extraction column at an intermediate pointtherein to introduce the said mixture into the countercurrent streams ofsaid immiscible solvent.

3. A process according to claim 2 comprising reuxing a part of thearomatic amine having the higher carbon to hydrogen ratio to the end ofthe column where the hydrocarbon solvent is introduced, and refluxing aportion of the aromatic amine having the lower carbon to hydrogen ratioto the opposite end of the column.

4. A process according to claim 1 in which the said dinitrile solvent isa dinitrile ether having the formula NC-Alk-O--Alk-CN wherein Alk is alower alkylene radical.

5. A process according to claim 4 in which the said dinitrile ethersolvent is bis(2cyanoethyl) ether.

6. A process according to claim 1 in which the said dinitrile solvent isa dinitrile sulfide having the formula NC--Alk-S-Alk-CN wherein Alk is alower alkylene radical.

7. A process according to claim 6 in which the said dinitrile suldesolvent is bis(2cyanoethyl) sulde.

8. A process according to claim 1 in which the said dinitrile solvent isa dinitrile amine having the formula wherein Alk is a lower alkyleneradical.

9. A process according to claim 8 in which the said dinitrile aminesolvent is bis(2-cyanoethyl) amine.

10. A process according to claim 1 in which the said aromatic aminemixture is a mixture of N- mono-lower alkyl and N,Ndi1ower alkyl aro- 10matic amies, the former being preferentially extracted by the dinitrilesolvent and the latter by the hydrocarbon solvent.

l1. A process according to claim 10 in which the said aromatic aminemixture is a mixture of N-monoethyl-aniline and N,Ndiethylaniline.

l2. A process according to claim 10 in which the said aromatic aminemixture is a mixture of N-ethyl-o-toludine and N,Ndiethy1otoludine.

13. A process according to claim 10 in which the said dinitrile solventis bis(2cyanoethyl) ether.

14. A process according to claim 1 in which the said aromatic aminemixture is a mixture of N,Ndiethy1aniline and p-ethylaniline.

15. A process according to claim 1 in which the said aromatic aminemixture is a mixture of nuclear lower alkyl-substituted aromatic aminesand nuclear unsubstituted aromatic amines.

16. A process according to claim 1 in which the said aromatic aminemixture is a mixture of pyridine and alkyl pyridines containing loweralkyl groups.

17. A process according to claim 1 in which the said aromatic aminemixture is a mixture of quinoline and alkyl quinolines containing loweralkyl groups.

18. A process according to claim 1 in which the said aromatic aminemixture is a mixture of heterocyclic aromatic amines having at least onenitrogen-containing ring and different carbon to hydrogen ratios andcontaining not more than two rings.

ALFRED G. HELL EUGENE C. MEDCALF. WILLIAM E. SISCO.

References Cited in the file of this patent UNITED STATES PATENTS NumberName Date 2,439,534 Wilkes Apr. 13, 1948 2,441,327 Maximus -..M May 18.1948

1. A METHOD OF SEPARATING AROMATIC AMINES HAVING DIFFERENT CARBON TOHYDROGEN RATIOS AND CONTAINING NOT MORE THAN TWO RINGS, WHICH COMPRISESCONTACTING A MIXTURE OF SAID AROMATIC AMINES WITH A MIXTURE OFIMMISCIBLE ORGANIC SOLVENTS FOR SAID AROMATIC AMINES, ONE OF SAIDIMMISCIBLE SOLVENTS BEING A DINITRILE SOLVENT HAVING THE FOLLOWINGFORMULA